1. Field of the Invention
This invention relates to apparatus for measuring the impedance of a low conductivity sample. In an important example, the apparatus can be used to monitor low level microbial growth where it is typically necessary to measure electrical conductivities in the range 0.001 to 30 mS/cm.sup.-1.
2. Description of Related Art
It is well established that measurement of the electrical impedance of bacteria suspended in a growth medium can provide useful information concerning bacterial growth. This has been done in some cases by inserting electrodes in the sample and making a direct impedance measurement. Advantages have been seen, however, in employing non-invasive methods and a number of oscillometric techniques have been suggested which can readily be operated in a non-invasive manner, that is to say with no physical contact between the sample and any electrode of the measuring system. In these oscillometric techniques, a sample of the medium is disposed in relation to an electrical resonant circuit in such a manner that the frequency of reasonance is indicative of the sample impedance.
The known techniques often fall into two categories: inductive techniques where the sample is placed within a measurement coil, and capacitive techniques in which the test sample is placed between electrodes to form a capacitor. Inductive oscillometers perform best with highly condcutive samples and are therefore used only rarely in this and related apllications. A number of different arangements of capacitive oscillometers have been proposed but difficulties have been found with existing arrangements, particularly in their resolution. It is believed that for microbial suspension studies, a resolution is desirable of 1 in 10.sup.-8 or better.